Optical glass

ABSTRACT

Optical glass with a path length independent of temperature includes SiO 2 , B 2  O 3 , K 2  O, BaO, ZrO 2 , TiO 2 , Nb 2  O 5 , P 2  O 5  and F in specified percentages by weight, with the sum of the amounts of SiO 2 , B 2  O 3  and P 2  O 5  between 38 and 45% by weight and the sum of the amounts of ZrO 2  and TiO 2  between 12 and 27% by weight.

This invention relates to optical glass.

It is known that wave fronts are deformed in a glass if the optical path-lengths are different in different portions of the glass as a result of temperature gradients.

Optical instruments, e.g. in space vehicles, are at times subject to strong temperature differences resulting from radiation from the sun and reflection in space, besides emitted and/or reflected infra-red radiation of nearby planets. These temperature differences can cause wavefront deformation in the optical glass, said deformation considerably reducing the resolving power of the optical systems. Even in aerial photography and in laser technology, the temperature gradients arising there can lead to considerable wavefront deformation.

There is therefore a great interest in the provision of optical systems and components made of glass with an optical path length independent of temperature.

In plane parallel lenses or plane parallel laser rods, the change in optical path length resulting from a change in temperature is: ##EQU1## IN WHICH: D = THICKNESS OF THE GLASS,

n = refractive index of the glass,

α = coefficient of thermal expansion, and

t = temperature.

A wavefront deformation is caused if ΔW is of different values at different portions of the optical component as a result of temperature gradients. From the above equation it can be seen that the optical path difference ΔW may be reduced by reducing the thickness d of the glass element component, the temperature difference Δt or the value G = (n-1)α + dn/dt. The value G is only dependent on the physical properties of the glass and should be as small as possible, 0 ideally.

The value of dn/dt is dependent on the summation of two components:

1. The volumetric expansion of the glass resulting from an increase in temperature causes a change towards small refractive indices, i.e. a negative dn/dt value.

2. The ultra-violet eigenfrequency λ_(o) of the glass is moved to greater wave lengths by an increase in temperature and as a result the refractive index is increased (positive dn/dt values).

In most glass the second influence is predominant, i.e. the dn/dt values are positive.

In order to obtain negative dn/dt values, components must be incorporated in the glass which:

A. keep the temperature dependence of the ultraviolet eigenfrequency dλ_(o) /dt as small as possible and/or

B. effect movement of the ultra-violet eigenfrequency λ_(o) to small wave lengths.

According to the coefficient of thermal expansion α is implicitly contained in the dn/dt value and there is accordingly a requirement for a high coefficient of thermal expansion α. According to the requirement that G = (n-1) × α + dn/dt = 0 on the other hand a α value which is as small as possible is desirable. This contrast shows the difficulties which arise in the development of glass with an optical path length that is independent of temperature.

It was found that the athermal wave aberration is not determined only by G but that the refractive index gradient resulting from thermal expansion, σ_(W), also determines the wave aberration. Therefore, the "classic G-value" must be supplemented by a value ##EQU2## so that complete athermality may be achieved. This value can be kept low if α, the modulus of elasticity and δn/δσof the glass are low. Generally ##EQU3## is positive so that, because of compensation, the "classic G-value" should be negative. The requirement for the "classic G-value" is thus nil or negative.

No flint glass in the optical property range

    n.sub.e 1.58 to 1.67, and v.sub.e < 44 to 30,

has previously been known which fulfils these conditions.

It is an object of the present invention to provide compositions for glass with low v-values (v_(e) < 44) (flint glass), in which the effects of a temperature gradient on the optical properties of the glass are minimized.

This object is achieved according to the invention with a glass which includes the following components in percentages by weight:

    ______________________________________                                         SiO.sub.2         6 to 25,                                                     B.sub.2 O.sub.3   15 to 36,                                                    K.sub.2 O         13 to 21,                                                    BaO               6 to 30,                                                     ZrO.sub.2         4 to 17,                                                     TiO.sub.2         6 to 10,                                                     Nb.sub.2 O.sub.5  4 to 20,                                                     P.sub.2 O.sub.5   0 to 12,   and                                               F                 6 to 12,                                                     ______________________________________                                    

wherein the sum of the amounts of SiO₂, B₂ O₃ and P₂ O₅ is between 38 and 45 weight %, the sum of the amounts of ZrO₂ and TiO₂ is between 12 and 27 weight %. In addition to the said oxides, the glass may include up to 7 weight % ZnO, up to 5 weight % CdO, PbO and/or up to 10% Y₂ O₃ and/or 5 weight % of 3-valent oxides, such as La₂ O₃, Al₂ O₃, Sb₂ O₃, and Bi₂ O₃, and of WO₃, which can be used for adjusting the optical properties and for stabilization of crystallization of the glass without the value:

    Ge.sub.abs × 10.sup.6 /° C. = 2.5

being exceeded and without the coefficient of expansion α × 10⁷ /° C. in the temperature range of -30° C. to 70° C. increasing above 120.

With the compositions according to the invention, a glass may be obtained with which the Ge_(abs) × 10⁶ value = 0. It has been found that the favorable effects of boric acid and fluorine on the eigen frequency λ_(o) are best retained, i.e. the athermal properties are not disadvantageously changed if ZrO₂, TiO₂ and Nb₂ O₅ are added in the stated amounts as shown in the following examples:

    ______________________________________                                         Example 1                                                                      SiO.sub.2                                                                              16.3 weight %                                                                               n.sub.e         = 1.5884                                  B.sub.2 O.sub.3                                                                        25.8 weight %                                                                               v.sub.e         = 39.90                                   K.sub.2 O                                                                              20.6 weight %                                                                               specific                                                                       gravity s       = 2.895                                   BaO      9.8 weight %                                                                               α × 10.sup.7 /° C.                                                          = 113                                                          -30 to 70° C.                                      ZrO.sub.2                                                                              14.0 weight %                                                          TiO.sub.2                                                                               8.8 weight %                                                                               Tg°  C.  = 392                                      Nb.sub.2 O.sub.5                                                                        4.7 weight %                                                                               ##STR1##        = -6.6                                   F       12.0 weight %                                                                               Ge.sub.abs × 10.sup.6 /° C.                                                       = 0                                                            modulus of                                                                     elasticity                                                                     [kp/mm.sup.2 ]  = 5343                                                         Poisson's ratio = 0.269                                   ______________________________________                                    

Partial exchange of ZrO₂ by Nb₂ O₅ is possible for achieving still lower v_(e) values without the Ge-value increasing as shown in Example 2:

    ______________________________________                                         Example 2                                                                      SiO.sub.2                                                                             16.3 weight %                                                                              n.sub.e           = 1.61450                                 B.sub.2 O.sub.3                                                                       25.8 weight %                                                                              v.sub.e           = 36.31                                   K.sub.2 O                                                                             20.6 weight %                                                                              s                 = 2.88                                    BaO     9.8 weight %                                                                              α × 10.sup.7 /° C.                                                            = 110.7                                                      -30 to 70° C.                                        ZrO.sub.2                                                                              4.0 weight %                                                                              Tg°  C.    = 448                                     TiO.sub.2                                                                              8.8 weight %                                                            Nb.sub.2 O.sub.5                                                                      14.7 weight %                                                                              ##STR2##          = -6.7                                   F      12.0 weight %                                                                              Ge.sub.abs × 10.sup.6 /° C.                                                         = 0.1                                                        modulus of                                                                     elasticity [kp/mm.sup.2 ]                                                                        = 5708                                    ______________________________________                                    

With further replacement of Nb₂ O₅ by BaO the Ge-value rises unexpectedly with a decreasing coefficient of expansion as shown in Example 3:

    ______________________________________                                         Example 3                                                                      SiO.sub.2                                                                              16.3 weight %                                                                              n.sub.e         = 1.61527                                  B.sub.2 O.sub.3                                                                        25.8 weight %                                                                              v.sub.e         = 39.85                                    K.sub.2 O                                                                              20.6 weight %                                                                              s               = 2.983                                    BaO     14.8 weight %                                                                              α × 10.sup.7 /° C.                                                          = 100.2                                                        -30 to 70° C.                                       ZrO.sub.2                                                                               9.0 weight %                                                                              Tg°  C.  = 434                                      TiO.sub.2                                                                               8.8 weight %                                                           Nb.sub.2 O.sub.5                                                                        4.7 weight %                                                                              ##STR3##        = -4.2                                    F       12.0 weight %                                                                              Ge.sub.abs × 10.sup.6 /° C.                                                       = +2.0                                     ______________________________________                                    

Inserting Na₂ O or Li₂ O instead of K₂ O is possible in small amounts <2 %, but unfavourable in larger proportions because the coefficient of expansion is very sharply increased. The more ready vaporization of the alkali components in the presence of fluorine adversely affects production of the glass because of selective surface vaporization which cause a lack of homogeneity.

With rising BaO content and decreasing fluorine content the coefficient of expansion becomes smaller, the refractive index increases and the Ge-value increases to +1.2, as shown in Example 4:

    ______________________________________                                         Example 4                                                                      SiO.sub.2                                                                              12.3 weight %                                                                              n.sub.e         = 1.60635                                  B.sub.2 O.sub.3                                                                        29.8 weight %                                                                              v.sub.e         = 42.60                                    K.sub.2 O                                                                              20.6 weight %                                                                              α × 10.sup.7 /° C.                                                          = 103.3                                                        -30 to 70° C.                                       BaO     19.8 weight %                                                           ZrO.sub.2                                                                               4.0 weight %                                                                              ##STR4##        = -5                                      TiO.sub.2                                                                               8.8 weight %                                                                              Ge.sub.abs × 10.sup.6 /° C.                                                       = +1.2                                     Nb.sub.2 O.sub.5                                                                        4.7 weight %                                                          F        5.0 weight %                                                          ______________________________________                                    

The sum of the components BaO, ZrO₂, TiO₂ and Nb₂ O₅ amounts to between 32 and 45 weight %, so that the fluorine proportion is ≧5.0 weight %. BaO can be replaced by SrO up to 20 weight % and by MgO up to 5 weight %. Care should, however, then be taken to ensure that the K₂ O content is not smaller than 13%, so that a Ge-value of + 2.5 × 10.sup.⁻⁶ /° C. is not exceeded. Ge-values <0 can be achieved by further exchange of SiO₂ by B₂ O₃ and inclusion of Al₂ O₃, see Examples 5 and 6, but then the tendency to crystallize is increased so strongly that larger objective components (300 mm. φ, 60 mm. thickness) cannot be produced. Compositions which are stable to crystallization have Ge-values between 0.5 and 1.2 × 10.sup.⁻⁶ /° C. (see Examples 4 and 7). The chemical resistance of the glass decreases with increasing BaO content. The compositions with BaO content ≦ 20 weight % are especially favorable.

The invention is illustrated in more detail by the following examples:

    ______________________________________                                         Example 5                                                                      SiO.sub.2                                                                               8.0 weight %                                                                              n.sub.e         = 1.61525                                  B.sub.2 O.sub.3                                                                        34.1 weight %                                                                              v.sub.e         = 34.88                                    K.sub.2 O                                                                              20.6 weight %                                                                              α × 10.sup.7 /° C.                                                          = 116.1                                                        -30 to 70° C.                                       BaO      9.8 weight %                                                           TiO.sub.2                                                                               8.8 weight %                                                                              ##STR5##        = -8.1                                    ZrO.sub.2                                                                               4.0 weight %                                                                              Ge.sub.abs × 10.sup.6 /° C.                                                       = -0.9                                     Nb.sub.2 O.sub.5                                                                       14.70 weight %                                                         F       12.0 weight %                                                          Example 6                                                                      SiO.sub.2                                                                              16.3 weight %                                                                              n.sub.e         = 1.58746                                  B.sub.2 O.sub.3                                                                        25.9 weight %                                                                              v.sub.e         = 37.11                                    K.sub.2 O                                                                              20.7 weight %                                                                              α × 10.sup.7 /° C.                                                          = 116.2                                                        -30 to 70° C.                                       BaO      9.8 weight %                                                           Al.sub.2 O.sub.3                                                                        4.5 weight %                                                                              ##STR6##        = -7.6                                    TiO.sub.2                                                                               8.8 weight %                                                                              Ge.sub.abs × 10.sup.6 /° C.                                                       = -0.7                                     ZrO.sub.2                                                                               4.0 weight %                                                          Nb.sub.2 O.sub.5                                                                       10.0 weight %                                                          F       12.0 weight %                                                          Example 7                                                                      SiO.sub.2                                                                              16.3 weight %                                                                              n.sub.e         = 1.60384                                  B.sub.2 O.sub.3                                                                        25.7 weight %                                                                              v.sub.e         = 39.42                                    K.sub.2 O                                                                              20.7 weight %                                                                              α × 10.sup.7 /° C.                                                          = 109.3                                                        -30 to 70° C.                                       BaO     14.8 weight %                                                           TiO.sub.2                                                                               8.8 weight %                                                                              ##STR7##        = -6.1                                    ZrO.sub.2                                                                               4.0 weight %                                                                              Ge.sub.abs × 10.sup.6 /° C.                                                       =  +0.5                                    Nb.sub.2 O.sub.5                                                                        9.7 weight %                                                          F       12.0 weight %                                                                              modulus of                                                                     elasticity                                                                     [kp/mm.sup.2 ]  = 5989                                                         Poisson's ratio = 0.267                                    ______________________________________                                    

The compositions of the glass according to the invention lie in the following area:

    ______________________________________                                                Oxides       Weight %                                                   ______________________________________                                                SiO.sub.2     6 to 25                                                          B.sub.2 O.sub.3                                                                             15 to 36                                                          P.sub.2 O.sub.5                                                                              0 to 12                                                          K.sub.2 O    13 to 21                                                          BaO           6 to 30                                                          ZrO.sub.2     4 to 17                                                          TiO.sub.2     6 to 10                                                          Nb.sub.2 O.sub.5                                                                             4 to 20                                                          F             5 to 12                                                          ZnO          0 to 7                                                            PbO          0 to 5                                                            Al.sub.2 O.sub.3                                                                            0 to 5                                                            La.sub.2 O.sub.3                                                                            0 to 5                                                            Bi.sub.2 O.sub.3                                                                            0 to 5                                                            Sb.sub.2 O.sub.3                                                                            0 to 5                                                            WO.sub.3     0 to 5                                                            Y.sub.2 O.sub.3                                                                              0 to 10                                                          Ta.sub.2 O.sub.5                                                                             0 to 10                                                   ______________________________________                                    

wherein ZnO, PbO, Al₂ O₃, La₂ O₃, Bi₂ O₃, Y₂ O₃ (Gd₂ O₃), Sb₂ O₃, Ta₂ O₅, WO₃ can be added to obtain desired optical properties and to stabilize crystallization without the Ge_(abs) × 10⁶ /° C. --value exceeding 2.5.

BaO can be replaced by SrO up to 15%, by CaO up to 8%, by MgO up to 5% and Nb₂ O₅ can be replaced by Ta₂ O₅ up to 10 weight %, if it is necessary to stabilize crystallization.

The proportions of fluorine in the said compositions generally act as a purifying means. However, As₂ O₃ as well as Sb₂ O₃ may be used as an additional purifying means. The fluorine replaces corresponding proportions of oxygen and is normally added as potassium fluoride or potassium bifluoride. Fluorine can, however, also be added as BaF₂ or as a fluoride of any of the other metals.

The addition of Li₂ O and Na₂ O in small amounts offers no special advantages other than that, as expected, the coefficient of expansion increases.

The glass according to the invention from this composition range fulfils the following conditions besides those already mentioned:

The coefficient of expansion ##EQU4## is smaller than 120.

A glass with a Ge_(abs) .sup.. 10⁶ /° C.--value > 0.5 is distinguished by especially high stability of crystallization which facilitates production of large-size optical components (minimum dimensions of 300 mm. φ, thickness 60 mm.).

The glass according to the invention has a good chemical resistance because of its high proportions of TiO₂, ZrO₂ and Nb₂ O₅.

The glass according to the invention has a positive relative partial dispersion ##EQU5## comparable with conventional flint glass of similar or the same optical properties (Table 3).

In Table 1 the said and other examples of composition are assembled and in Table 2 the corresponding physical properties are set out.

    ______________________________________                                                 Production of a 6 l. melt                                              n.sub.e = 1.60635 v.sub.e = 42.60                                               α × 10.sup.7 /° C. = 103.3                                                   Ge.sub.abs × 10.sup.6 /° C. = 1.23              Composition       Weight of Material                                           Oxides   Weight %     Raw Materials                                                                              kg.                                          ______________________________________                                         SiO.sub.SiO.sub.2                                                                       12.31        SiO.sub.2   4.932                                        B.sub.2 O.sub.3                                                                         29.88        H.sub.2 BO.sub.3                                                                           21.153                                       BaO      5.76         Ba(NO.sub.3).sub.2                                                                         3.948                                        BaO      14.00        BaCO.sub.3  7.164                                        K.sub.2 O                                                                               14.36        KNO.sub.3   8.447                                        K.sub.2 O                                                                               6.20         KHF.sub.2   4.126                                        TiO.sub.2                                                                               8.84         TiO.sub.2   3.552                                        ZrO.sub.2                                                                               3.98         ZrO.sub.2   1.603                                        Nb.sub.2 O.sub.5                                                                        4.67         Nb.sub.2 O.sub.5                                                                           1.871                                        F        5.00                                                                  ______________________________________                                    

The well-mixed batch is introduced into a platinum melting pot and heated at 1160° to 1220° C. for about 8 hours; it is then purified for 2 hours at 1280° C. and for 1 hour at 1100° C. It is then stirred for 11/2 hours while cooling from 1100° C. to 920° C. and allowed to run off in pre-heated shapes.

The glass blocks are cooled from 450° C. as desired at a rate of 6° C./ hour or more slowly.

                                      Table 1                                      __________________________________________________________________________     Oxides                                                                              1   2   3   4   5   6   7   8   9   10  11  12                            __________________________________________________________________________     SiO.sub.2                                                                           16,3                                                                               16,3                                                                               16,3                                                                               12,3                                                                                8,0                                                                               16,3                                                                               16,3                                                                                6,3                                                                               9,3 16,3                                                                               16,3                                                                               16,3                          B.sub.2 O.sub.3                                                                     25,8                                                                               25,8                                                                               25,8                                                                               29,8                                                                               34,1                                                                               25,9                                                                               25,7                                                                               35,8                                                                               32,8                                                                               25,9                                                                               25,9                                                                               25,8                          P.sub.2 O.sub.5                                                                     --  --  --  --  --  --  --  --  --  --  --  --                            K.sub.2 O                                                                           20,6                                                                               20,6                                                                               20,6                                                                               20,6                                                                               20,6                                                                               20,7                                                                               20,7                                                                               20,6                                                                               20,6                                                                               20,7                                                                               20,7                                                                               14,6                          BaO   9,8                                                                                9,8                                                                               14,8                                                                               19,8                                                                                9,8                                                                                9,8                                                                               14,8                                                                                9,8                                                                               14,8                                                                                9,8                                                                                9,8                                                                               25,7                          ZnO  --  --  --  --  --  --  --  --  --  --  --  --                            PbO  --  --  --  --  --  --  --  --  --  --  --  --                            Al.sub.2 O.sub.3                                                                    --  --  --  --  --   4,5                                                                               --  --  --  --  --  --                            La.sub.2 O.sub.3                                                                    --  --  --  --  --  --  --  --  --   4,5                                                                               --  --                            Y.sub.2 O.sub.3                                                                     --  --  --  --  --  --  --  --  --  --  --  --                            ZrO.sub.2                                                                           14,0                                                                                4,0                                                                                9,0                                                                                4,0                                                                                4,0                                                                                4,0                                                                                4,0                                                                                4,0                                                                               14,0                                                                               4,0  4,0                                                                                4,0                                                                                4,0                      TiO.sub.2                                                                            8,8                                                                                8,8                                                                                8,8                                                                                8,8                                                                                8,8                                                                                8,8                                                                                8,8                                                                                8,8                                                                               8,8  8,8                                                                                8,8                                                                                8,8                          Ta.sub.2 O.sub.5                                                                    --  --  --  --  --  --  --  --  --  --   4,5                                                                               --                            Nb.sub.2 O.sub.5                                                                     4,7                                                                               14,7                                                                                4,7                                                                                4.7                                                                               14,7                                                                               10,0                                                                                9,7                                                                                4,7                                                                               9,7 10,0                                                                               10,0                                                                                4,8                          F    12,0                                                                               12,0                                                                               12,0                                                                                5.0                                                                               12,0                                                                               12,0                                                                               12,0                                                                               12,0                                                                               6,0  8.0                                                                               12,0                                                                               12,0                          __________________________________________________________________________     Oxides                                                                              13  14  15  16  17  18  19                                                __________________________________________________________________________     SiO.sub.2                                                                           16,3                                                                               16,2                                                                               16,3                                                                               15,2                                                                               16,3                                                                               23,0                                                                               16,3                                              B.sub.2 O.sub.3                                                                     25,8                                                                               25,7                                                                               25,7                                                                               24,2                                                                               25,9                                                                               18,6                                                                               15,0                                              P.sub.2 O.sub.5                                                                     --  --  --  --  --  --  10,8                                              K.sub.2 O                                                                           20,6                                                                               14,2                                                                               20,7                                                                               19,3                                                                               16,6                                                                               20,4                                                                               20,6                                              BaO   8,8                                                                               16,0                                                                                7,3                                                                                9,2                                                                               14,8                                                                               20,6                                                                                9,8                                              ZnO  --  --  --   6,8                                                                               --  --  --                                                PbO  --  --  --  --   4,0                                                                               --  --                                                Al.sub.2 O.sub.3                                                                    --  --  --  --  --  --  --                                                La.sub.2 O.sub.3                                                                    --  --  --  --  --  --  --                                                Y.sub.2 O.sub.3                                                                      6,0                                                                               --  --  --  --  --  --                                                ZrO.sub.2                                                                            4,0                                                                                4,4                                                                               16,5                                                                                4,0                                                                                4,0                                                                                4,0                                                                                4,0                                              TiO.sub.2                                                                            8,8                                                                                8,8                                                                                8,8                                                                                8,2                                                                                8,8                                                                                8,8                                                                                8,8                                              Ta.sub.2 O.sub.5                                                                    --  --  --  --  --  --  --                                                Nb.sub.2 O.sub.5                                                                     9,7                                                                               14,7                                                                                4,7                                                                               13,7                                                                                9,6                                                                                4,6                                                                               14,7                                              F    12,0                                                                               12,0                                                                               12,0                                                                               12,0                                                                               12,0                                                                               12,0                                                                               12,0                                              __________________________________________________________________________

                                      Table 2                                      __________________________________________________________________________     Physical                                                                       Properties                                                                             1    2    3    4    5    6    7    8    9                              __________________________________________________________________________     n.sub.e 1,58842                                                                             1,61450                                                                             1,61527                                                                             1,60381                                                                             1,61525                                                                             1,58746                                                                             1,60384                                                                             1,59657                                                                             1,61386                        v.sub.e 39,7 35,7 39,9 43,0 34,9 37,1 39,4 38,3 39,4                           s       2,845                                                                               2,882                                                                               2,983                                                                               2,962                                                                               2,839                                                                               2,779                                                                               2,942                                                                               2,810                                                                               2,877                          α .sup.. 10.sup.7 /° C                                                    113,3                                                                               110,7                                                                               100,2                                                                               104,4                                                                               110,0                                                                               116,2                                                                               109,3                                                                               118,2                                                                               108,4                          -10 bis + 70° C                                                         Tg° C                                                                           392  399  434  443  375  381  421  389  428                             ##STR8##                                                                               -6,6                                                                                -6,7                                                                                -4,2                                                                                -5,3                                                                                -7,4                                                                                -7,6                                                                                -6,1                                                                                -7,5                                                                                -5,5                          Ge.sub.abs .sup.. 10.sup.6 /° C                                                 ±0                                                                               +0,1 +2,0 +1,0 -0,7 -0,7 +0,5 -0,5 +1,2                           __________________________________________________________________________     Physical                                                                       Properties                                                                             10   11   12   13   14   15   16   17   18   19                        __________________________________________________________________________     n.sub.e 1,61657                                                                             1,60773                                                                             1,60877                                                                             1,61376                                                                             1,65105                                                                             1,60720                                                                             1,63323                                                                             1,61129                                                                             1,59532                                                                             1,63595                   v.sub.e 38,9 38,0 42,4 39,3 34,5 38,5 36,4 38,6 44,1 34,9                      s       2,926                                                                               2,916                                                                               3,13 2,898                                                                               3,059                                                                               2,862                                                                               3,002                                                                               3,002                                                                               3,047                                                                               2,980                     α .sup.. 10.sup.7 /° C                                                    105,5                                                                               110,9                                                                               101,6                                                                               102,4                                                                               92,1 103,9                                                                               99,9 109,2                                                                               105,9                                                                               110,0                     -10 bis +70° C                                                          Tg° C                                                                           437  406  438  442  444  416  421  413  456  454                        ##STR9##                                                                               -4,8                                                                                -5,8                                                                                -5,1                                                                                -4,1                                                                                -3,5                                                                                -4,9                                                                                -3,2                                                                                -5,8                                                                                -5,0                                                                                -5,5                     Ge.sub.abs .sup.. 10.sup.6 /° C                                                 +1,7 +0,9 +1,1 +2,2 +2,5 +1,4 +2,5 +0,9 +1,3 +1,5                      __________________________________________________________________________

                                      Table 3                                      __________________________________________________________________________      Type                                                                               (Example)                                                                            n.sub.d                                                                              v.sub.d                                                                            PgF                                                                                ##STR10##                                                                              Ge.sub.abs × 10.sup.6 /°         __________________________________________________________________________                                     C                                              F 5        1,603420                                                                            38,03                                                                              0,5790                                                                             +3,3    +8,1                                           F 8        1,595510                                                                            39,18                                                                              0,5772                                                                             +3,3    +8,2                                               (15)  1,60182                                                                              38,65                                                                              0,5868                                                                             -4,9    +1,4                                           TiF 5     1,59355                                                                              35,51                                                                              0,5928                                                                             -1,2    +4,2                                               (2)   1,60809                                                                              36,41                                                                              0,5936                                                                             -6,7    +0,1                                           LF 2      1,58921                                                                              40,94                                                                              0,5746                                                                             +0.9    +6,2                                           LF 5       1,581440                                                                            40,85                                                                              0,5745                                                                             +1,5     +6,79                                             (1)   1,58479                                                                              40,14                                                                              0,5845                                                                             -6,6     0                                             F 8       1,59551                                                                              39,18                                                                              0,5772                                                                             +3,3    +8,2                                               (7)   1,60020                                                                              39,67                                                                              0.5844                                                                             -6,1    +0,5                                           BaSF 3    1,60717                                                                              40,29                                                                              0,5762                                                                             +3,0    +7,8                                               (3)   1,61156                                                                              40,15                                                                              0,5807                                                                             -4,2    +2,0                                           __________________________________________________________________________ 

We claim:
 1. Optical flint glass with a path length substantially independent of temperature in the range n_(e) 1.58 to 1.66, v_(e) 44 to 30, the value Ge_(abs) × 10⁶ /° C. not exceeding 2.5 and the coefficient of thermal expansion α × 10⁷ /° C. in the temperature range of -30° C. to 70° C. not being above 120, said glass consisting essentially of the following components in weight %:

    ______________________________________                                         SiO.sub.2         6      to 25                                                 B.sub.2 O.sub.3   15     to 36                                                 K.sub.2 O         13     to 21                                                 BaO               6      to 30                                                 ZrO.sub.2         4      to 17                                                 TiO.sub.2         6      to 10                                                 Nb.sub.2 O.sub.5  4      to 20                                                 P.sub.2 O.sub.5   0      to 12                                                 F                 5      to 12                                                 ZnO               0      to  7                                                 PbO               0      to  5                                                 Al.sub.2 O.sub.3  0      to  5                                                 La.sub.2 O.sub.3  0      to  5                                                 Bi.sub.2 O.sub.3  0      to  5                                                 Sb.sub.2 O.sub.3  0      to  5                                                 WO.sub.3          0      to  5                                                 Y.sub.2 O.sub.3   0      to 10                                                 Ta.sub.2 O.sub.5  0      to 10,                                                ______________________________________                                    

wherein said F replaces corresponding proportions of oxygen, wherein the sum of the amounts of SiO₂, B₂ O₃ and P₂ O₅ is between 38 and 45 weight % and the sum of the amounts of ZrO₂ and TiO₂ is between 12 and 27 weight %, and wherein ZnO, PbO, Al₂ O₃, La₂ O₃, Bi₂ O₃, Y₂ O₃ Gd₂ O₃, Sb₂ O₃, Ta₂ O₅, WO₃ can be added to obtain desired optical properties and to stabilize crystallization without the Ge_(abs) × 10⁶ /° C. --value exceeding 2.5, BaO can be replaced by SrO up to 15%, by CaO up to 8%, by MgO up to 5% and Nb₂ O₅ can be replaced by Ta₂ O₅ up to 10 weight %, and wherein K₂ O can be replaced by Na₂ O or Li₂ O by up to 2 weight %.
 2. Optical glass according to claim 1, which consists essentially of:

    ______________________________________                                         SiO.sub.2    16.3 weight %                                                     B.sub.2 O.sub.3                                                                             25.8 weight %                                                     K.sub.2 O    20.6 weight %                                                     BaO          9.8 weight %                                                      ZrO.sub.2    14.0 weight %                                                     TiO.sub.2    8.8 weight %                                                      Nb.sub.2 O.sub.5                                                                            4.7 weight %                                                      F            12.0 weight %                                                     ______________________________________                                    

said glass having the following properties:

    ______________________________________                                         n.sub.e                  = 1.5884                                              v.sub.e                  = 39.90                                               specific                                                                       gravity s                = 2,895                                               α × 10.sup.7 /° C.                                                                   = 113                                                 -30 to 70° C.                                                           Tg°  C.           = 392                                                  ##STR11##                = -6.6                                               Ge.sub.abs × 10.sup.6 /° C.                                                                = 0                                                   modulus of elasticity                                                          [kp/mm.sup.2 ]           = 5343                                                Poisson's ratio          = 0.269.                                              ______________________________________                                    


3. Optical glass according to claim 1, which glass consists essentially of:

    ______________________________________                                         SiO.sub.2    16.3 weight %                                                     B.sub.2 O.sub.3                                                                             25.8 weight %                                                     K.sub.2 O    20.6 weight %                                                     BaO          9.8 weight %                                                      ZrO.sub.2    4.0 weight %                                                      TiO.sub.2    8.8 weight %                                                      Nb.sub.2 O.sub.5                                                                            14.7 weight %                                                     F            12.0 weight%                                                      ______________________________________                                    

said glass having the following properties:

    ______________________________________                                         n.sub.e                  = 1.61450                                             v.sub.e                  = 36.31                                               α × 10.sup.7 /° C.                                                                   = 110.7                                               -30 to 70° C.                                                           Tg°  C.           = 448                                                  ##STR12##                = -6.7                                               Ge.sub.abs × 10.sup.6 /° C.                                                                = +0.1                                                modulus of elasticity                                                          [kp/mm.sup.2 ]           =
 5708.                                               ______________________________________                                    


4. Optical glass according to claim 1, which glass consists essentially of:

    ______________________________________                                         SiO.sub.2    16.3 weight %                                                     B.sub.2 O.sub.3                                                                             25.8 weight %                                                     K.sub.2 O    20.6 weight %                                                     BaO          14.8 weight %                                                     ZrO.sub.2    9.0 weight %                                                      TiO.sub.2    8.8 weight %                                                      Nb.sub.2 O.sub.5                                                                            4.7 weight %                                                      F            12.0 weight %                                                     ______________________________________                                    

and which has the following properties:

    ______________________________________                                         n.sub.e                 = 1.61527                                              v.sub.e                 = 39.85                                                s                       = 2.983                                                α × 10.sup.7 /° C.                                                                  = 100.2                                                -30 to 70° C.                                                           Tg°  C.          = 434                                                   ##STR13##              = -4.2                                                 Ge.sub.abs × 10.sup.6 /° C.                                                               = +2.0.                                                ______________________________________                                    


5. Optical glass according to claim 1, which glass consists essentially of:

    ______________________________________                                         SiO.sub.2    12.3 weight %                                                     B.sub.2 O.sub.3                                                                             29.8 weight %                                                     K.sub.2 O    20.6 weight %                                                     BaO          19.8 weight %                                                     ZrO.sub.2    4.0 weight %                                                      TiO.sub.2    8.8 weight %                                                      Nb.sub.2 O.sub.5                                                                            4.7 weight %                                                      F            5.0 weight %                                                      ______________________________________                                    

and which has the following properties:

    ______________________________________                                         n.sub.e                 = 1.60653                                              v.sub.e                 = 42.60                                                α × 10.sup.7 /° C.                                                                  = 103.3                                                -30 to 70° C.                                                            ##STR14##              = -5                                                   Ge.sub.abs × 10.sup.6 /° C.                                                               = +1.2.                                                ______________________________________                                    


6. Optical glass according to claim 1 stable to crystallization and having a Ge value above +0.5 × 10⁶ /° C.
 7. Optical glass according to claim 1 having a sufficiently high percentage of B₂ O₃ replacing SiO₂ and/or the inclusion of Al₂ O₃ to yield a Ge value less than
 0. 8. Optical glass according to claim 7 containing Al₂ O₃.
 9. Optical glass according to claim 7 containing none of SrO, CaO or MgO.
 10. Optical glass according to claim 7 containing none of Li₂ O or Na₂ O.
 11. Optical glass according to claim 7 containing none of P₂ O₅, ZnO, PbO, Al₂ O₃, La₂ O₃, Bi₂ O₃, Sb₂ O₃, WO₃, Y₂ O₃ or Ta₂ O₅. 